The objective of this research is to determine the mechanism by which pp60(v-src) suppresses the differentiation of myogenic progenitors to mature myotubes. We have identified an integrin based adhesion system which supplies a critical signal in this process and determined that this adhesion system is affected by pp60(v-src). In the present proposal the emphasis is on linking these effects to the transcriptional changes which are effected by pp60(v-src) through the analysis of the muscle specific transcription complexes in tsLA24A Rous sarcoma virus infected avian myoblasts maintained at permissive and non-permissive temperatures. This focuses on E box binding complexes which contain myoD, myf5, or myogenin and will involve biochemical analysis, analysis of phosphorylation of elements in the complexes, and functional analysis through the use of reporter gene constructs. Reversal of the developmental block will be analyzed using retroviral vectors which express myoD, myogenin, myf5, or id. The possible role of elements which are common to a number of differentiation systems and could provide a mechanism for the effects of pp60(v-src) on these systems will be examined through the analysis of the broadly expressed factors id and E12/E47. In addition to the avian system we will develop an analogous system based on a transgenic mouse strain which carries a tsLA90 v-src gene. This will allow us to expand the generality of the results and to test myoblasts derived from different stages of development. It will also provide the basis for a model system for examination of the v-src effects in vivo.